Methods of forming particulate films and films and devices made therefrom

ABSTRACT

A method of depositing a film comprising a monolayer of particles. The method includes providing a dispersion comprising particles and at least two liquids and depositing drops of the dispersion onto a substrate and evaporating the at least two liquids resulting in a film of a monolayer of the particles. One embodiment of the method includes a coating on the outer surface of particles such that the coating makes the particles substantially non-dispersible, substantially non-soluble and substantially non-suspendable in one of the liquids. A particulate film containing at least one layer of particles, wherein the at least one layer is substantially made of particles of a chemical composition and has uniform thickness. Optical devices containing a particulate film containing at least one layer of particles, wherein the at least one layer is substantially made of particles of a chemical composition and has uniform thickness.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present U.S. patent application is a divisional of U.S. patentapplication Ser. No. 15/429,238 filed Feb. 10, 2017, which is related toand claims the priority benefit of U.S. Provisional Patent ApplicationSer. No. 62/294,659 filed Feb. 12, 2016, the contents of which arehereby incorporated by reference in their entirety into the presentdisclosure.

TECHNICAL FIELD

This disclosure relates to methods of forming particulate filmsincluding monolayer films. This disclosure also relates to devices madefrom such particulate films.

BACKGROUND

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, these statements are to beread in this light and are not to be understood as admissions about whatis or is not prior art.

There are many industrial applications such as but not limitedoptoelectronics, electronic devices, sensors, and photovoltaicapplications where uniform films of particles is required. By uniform,we mean uniform thickness such that variations across a particulate filmare of the order of magnitude of the size of the particles comprisingthe film. Techniques used for this in the industry include Langmuirtransfer techniques which is a vacuum-based technique. Other methodsinclude chemical vapor deposition (CVD) and physical vapor deposition,which require complex equipment and have higher energy cost.

Thus, there exists a need for techniques for forming uniform films ofparticles that require no vacuum and no transfer techniques and arecapable of forming films directly onto a desired substrate without theneed for complex equipment and high energy cost.

SUMMARY

A method of forming a film comprising a monolayer of particles isdisclosed. The method includes providing a dispersion comprisingparticles and at least two liquids, depositing drops of the dispersiononto a substrate, and evaporating the at least two liquids, whereincomplete evaporation of the at least two liquids results in formation afilm comprising a monolayer of the particles.

A method of forming a film comprising particles is disclosed. The methodincludes providing particles with a coating on their outer surface suchthat the coating makes the particles substantially non-dispersible,substantially non-soluble and substantially non-suspendable in a firstliquid, but are substantially soluble and dispersible in a secondliquid, wherein the first liquid and the second liquid are miscible andthe evaporation rate of the second liquid is greater than theevaporation rate of the first liquid; dispersing the particles into thesecond liquid and mixing the resulting mixture with the first liquid toform a process mixture; depositing the process mixture onto a substrate;and evaporating the first liquid and second liquid to form a filmcomprising the particles on the substrate.

A particulate film containing at least one layer of particles isdisclosed. The at least one layer of the particular film issubstantially made of particles of a chemical composition and hasuniform thickness.

An optical device which includes a particulate film containing at leastone layer of particles is disclosed. The at least one layer of theparticulate film of the optical device is substantially made ofparticles of a chemical composition and has uniform thickness.

BRIEF DESCRIPTION OF DRAWINGS

Some of the figures shown herein may include dimensions. Further, someof the figures shown herein may have been created from scaled drawingsor from photographs that are scalable. It is understood that suchdimensions or the relative scaling within a figure are by way ofexample, and not to be construed as limiting.

FIG. 1A is a schematic representation of a drop of an evaporatingdispersion containing thiol-capped Ga—In particles in an ethanol/waterdual-solvent mixture.

FIG. 1B shows an image of the drop from the suspension as describedabove at an early stage of evaporation (<10% of evaporation time).

FIG. 1C is in image of the middle stage of evaporation (50% ofevaporation time) of the drop.

FIG. 1D is an image of the drop after evaporation.

FIG. 2A shows profile images of sessile drops with various particleconcentrations at different times during the evaporation process.

FIGS. 2B and 2C are plots of contact angle and volume of the dropagainst evaporation time respectively.

FIGS. 2D and 2E are plots of dimensionless contact angle dimensionlessvolume against time normalized by drop size and initial contact angle.

FIGS. 3A and 3B summarizes the influence of initial particleconcentration on the final morphology of the film.

FIGS. 4A and 4B are qualitative examples demonstrating use of films ofthis disclosure as thin film devices.

FIG. 4C is a representative SEM image of film morphology for the 84±18nm samples used in FIGS. 4A and 4B.

FIGS. 4D through 4G show the measured optical results (for variousassembled films of various particle sizes d in nm, namely absorbance,transmittance, reflectance and specular reflectance normalized by totalreflectance respectively.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of thedisclosure, reference will now be made to the embodiments illustrated inthe drawings and specific language will be used to describe the same. Itwill nevertheless be understood that no limitation of the scope of thedisclosure is thereby intended.

Particles can be dissolved or suspended in a liquid. In many instances,the particles can dissolve to certain degree in the liquid, which we cancall a solvent. In this disclosure, for simplicity, we use liquid andsolvent interchangeably while recognizing that solubility of aparticular material in a particular liquid is near-zero or negligible.In several instances in addition to dissolved particles there can besuspended particles either because dissolution limit has been reached orthe conditions are not favorable for further dissolution or both. Insuch a case, the undissolved particles are dispersed in the solvent. Forpurposes of this disclosure such solvent+particle mixtures are termed asdispersions. From such dispersions, as a drop of liquid evaporates on asolid surface, the particles within form a film. Due to the myriadphysical and chemical processes involved during evaporation, thestructure of this film can vary greatly, ranging from simple ringformations like those seen in coffee drops to more complex morphologiessuch as fractal patterns. Of these possible outcomes, a strikinglyuniform film results by using dispersions with multiple solvents. Thesefilms also show great potential for industrial applications, as they mayprovide simpler, faster, and less costly ways to manufacture thin filmdevices compared to conventional methods. By appropriately tuning designparameters, multiple-solvent based films could also be used to produceconformal monolayer coatings on three dimensional surfaces or extremelyfine hole patterns. Here we ascribe the uniform films associated withdispersions containing multiple solvents to a hybrid self-assemblyprocess.

The methods, films and devices of this disclosure are furtherillustrated with a non-limiting example of a two-solvent dispersion. Inthis disclosure such a dispersion is also called mixture or suspension.A drop from the two-solvent dispersion segregates during evaporation,resulting in an elevated concentration of higher vapor pressure solventat the drop surface. Consequently the particles, miscible only in thehigher vapor pressure solvent, are carried toward the drop surface. Onceat the drop surface, the particles self-assemble due to attractiveparticle-interface interactions. Real-time observations duringevaporation reveal the mechanisms of this self-assembly process. Theobserved drop profile kinetics agree well with an evaporation model fordrops of dual-solvent dispersions developed in work leading to thisdisclosure. In experiments leading to this disclosure it has beendemonstrated that film uniformity depends on an optimal initial particleconcentration, below which voids result and above which exhibits acoffee ring formation occurs. This technique, as later described in thisdetailed description, has been is used in experiments leading to thisdisclosure to produce thin film devices such as flexible broadbandneutral density filters and semi-transparent mirrors. This hybridself-assembly approach requires no particle-substrate interactions, isscalable, robust, and transferrable. This disclosure describes methodsof uniform films based on dispersions containing two or more solventsand demonstrates applications for drop-based fabrication of thin filmdevices.

For purposes of this disclosure, the following nomenclature is employed.The word particles has its usual meaning. This disclosure refers toparticles in at least three different stages in the methods andembodiments of this disclosure: particles prior to dissolving ordispersing in a solvent; particles dissolved or dispersed in a solventor particles just added to a liquid; and particles that are constituentsof a film made by the methods of this disclosure. The term ‘deposit” isused for a drop or collection of drops of dispersion deposited on asubstrate. When the liquid content of the deposit evaporates leavingbehind a collection of particles, this collection of particles is calleda film, for purposes of this disclosure.

This disclosure describes, as an example of utilizing a dispersioncontaining more than one solvent, a process that is applicable to anytwo-solvent system where the dispersed particles (or non-volatilematerial, in general) are miscible only in the solvent with a highervapor pressure. For purposes of this disclosure, a mixture of twosolvents for the particles is called a dual-solvent mixture.Illustratively, ethanol and water form a dual-solvent mixture for usewith thiol-capped Ga—In particles. Several aspects of this disclosurewill now be described in terms of this system through the use of FIGS.1A through 1D. FIG. 1A is a schematic representation of a drop 101 of adispersion containing thiol-capped eutectic Ga—In particles in anethanol/water dual-solvent system placed on a substrate 102. Referringagain to FIG. 1A, 103 is a schematic representation of thiol-cappedGa—In single nanoparticle covered with a self-assembled monolayer ofthiol. Thiol is shown as strands emanating from the Ga—In particle andits molecular structure is schematically represented as 104 in FIG. 1A.The arrows shown emanating from the surface of the drop 101 indicate thedirection and magnitude of the flux of an evaporating solvent in thedispersion. For illustrative purposes the arrows represent the flux ofone of the solvents. A similar set of arrows can be drawn for the othersolvent (or solvents in case of a multi-solvent dispersion) in thedual-solvent dispersion. In experiments leading to this disclosure,polydimethylsiloxane (PDMS) substrates were used to demonstrate theself-assembly process but the process is applicable to any substrate onwhich there is contact line pinning throughput evaporation.Single-solvent (ethanol only) drops exhibited various particlesize-dependent structures, including distributed clumps for largeparticles (268±51 nm), highly pronounced coffee rings for mediumparticles (110±22 nm), or less pronounced outer coffee rings withinterior fractal-like films for small particles (84±18 nm). It should benoted that fractal patterns were observed at the contact line forsingle-drops containing all particle sizes. With the inclusion of alower vapor pressure solvent (water in this case), the resulting filmstructures became strikingly uniform, ranging from thick, in the caseswhere the contact line becomes de-pinned early during the evaporationprocess, to extremely thin, when the contact line remains pinnedthroughout evaporation. This disclosure mainly addresses the latter caseof an extremely thin monolayer.

FIG. 1B shows an image of the drop from the suspension as describedabove at an early stage of evaporation (<10% of evaporation time). Forpurposes of this disclosure, evaporation time is the total amount oftime taken for all of the solvents to leave the drop. Referring to thesketch in FIG. 1B, it can be seen that assembly begins at the onset ofevaporation when a ring of nanoparticles nucleates at the contact line,where the flux of solvent leaving the drop is the highest. This isbecause particles within evaporating drops tend toward the surface whereevaporation flux is the highest. In the sketch in FIG. 2B, the arrowshave the same meaning as in FIG. 1A. Following nucleation, rather thanfurther accumulation of nanoparticles at the boundary to form apreviously observed coffee ring, the structure transitions into auniform sheet over the surface of the drop as the surface area isreduced and more nanoparticles are introduced to the interface. Notethat the Marangoni flow within the drop during evaporation recirculatesnanoparticles below the surface until they are introduced to theinterface. This Marangoni flow is a result of surface tension gradientsestablished by drop cooling effects, as well as nanoparticleconcentration gradients at the surface (i.e., the areas of the dropsurface that are densely packed with nanoparticles (initially at thecontact line) will exhibit a lower surface tension compared to areasthat are particle free, resulting in a surface flow from the edge of thedrop to its center). The direction of the Marangoni flow can be deducedby considering the influence of gradients in temperature, ethanolconcentration, and nanoparticle concentration on surface tensiongradients The Marangoni flow may be either thermally driven orconcentration driven, where the dominant case dictates the direction ofthe internal droplet flow. The mismatch in thermal conductivitiesbetween the substrate and the liquid indicates a Marangoni flow from thetop of the drop to the bottom along the liquid-air interface and fromthe contact line toward the center of the drop along the liquid-solidinterface and a corresponding thermal Marangoni number, MaT=5.18×10³. Onthe other hand, given the higher evaporation rate of ethanol compared towater, it is expected that there will be an elevated concentration ofethanol at the contact line where the flux is the highest. Becauseethanol also has a lower surface tension, this results in aconcentration driven surface tension gradient opposing the thermallydriven gradient and a flow opposing the one brought about by thermalgradients. The magnitude of the concentration driven Marangoni numberassociated with the gradients in ethanol concentration is MaC=1.53×10⁷,which is 4 orders of magnitude larger than MaT and therefore dominatesthe direction of the flow. Moreover, the influence of the nanoparticlescan further contribute to this flow in two ways. First, the areas of thedrop surface that are densely packed with nanoparticles (initially atthe contact line) will exhibit a lower surface tension compared to areasthat are particle free resulting in a surface flow from the edge of thedrop to its center along the liquid-air interface. Second, thenanoparticles can be viewed effectively as surface contaminants, whichcan further reduce MaT by as much as 2 orders of magnitude, thusincreasing the relative dominance of the concentration driven flow.

FIG. 1C is in image of the middle stage of evaporation (approximately50% of evaporation time) of the drop. It can be inferred from FIG. 1Cthat the monolayer growth at the drop surface continues throughout theevaporation process until the higher vapor pressure solvent hascompletely evaporated and all of the particles have reached theinterface. FIG. 1D is an image of the drop after evaporation. It can beseen from FIG. 1D that during the final stages of the evaporationprocess, the contact line of the evaporating fluid de-pins, allowing forthe assembled sheet of nanoparticles to collapse onto the substrate.These observations suggest a process qualitatively different frompreviously reported methods for producing uniform films from evaporatingdrops, which solely employ interparticle capillary interactions,particle-interface interactions, or fluid-particle interactions.Instead, this assembly process is a hybrid case wherein particle-fluidinteractions to carry the particles to the boundary, followed byparticle-interface interactions that assemble the particles into a sheetat the surface of the drop. The observed rigidity of the sheet isindicative of a solid-like nanocrystal. The sketch in FIG. 1C has thesame meaning in FIG. 1A. The sketch in FIG. 1D has the same meaning butsince the evaporation is complete, the flux of solvents is representedas near zero and there is no Marangoni flow.

In experiments leading to this disclosure, EGaInNP (In this disclosure,EGaInNP stands for Eutectic Gallium Indium Nano aprtciles} dispersionswere prepared by an ultrasonication method described in literature. Allsamples were prepared in 3 dram bottles (Kimble Chase). Beforeultrasonication the vials were thoroughly washed with Liquinox detergent(Alconox), followed by a rinse with distilled water, followed by a rinsewith ethanol to remove surface impurities. Once clean, a nominal mass of1 g of eutectic Gallium-Indium alloy (EGaIn) (Sigma-Aldrich) was addedto the vial via syringe. Masses were measured using a Ohaus Pioneerbalance with 0.1 mg resolution. This was followed by the addition of 120μL (Labnet BioPette, 1000 μL size) of 100 mM solution of3-mercapto-N-nonylpropionamide (1ATC9, Sigma-Aldrich). An additional3880 μL of pure ethanol was added to achieve a final thiol concentrationof 3 mM and final liquid volume of 4 mL. Once the sample was prepared,the liquid metal was dispersed into the solvent using a Qsonica Q700probe ultrasonicator fitted with a 1/16 in microtip. The vial with EGaInand solvent was contained in a water bath to prevent overheating andevaporation of the sample. The sample was sonicated for 1 h at anamplitude of 30%. After sonication, samples were either furtherprocessed via centrifugation or placed in a −35° C. freezer for laterprocessing. Differential centrifugation was used to separate particlesof various sizes. The process is as follows. Sonicated samples weredistributed evenly into 1.5 mL polypropylene centrifuge tubes, lightlybathsonicated (Branson 1800) in isopropyl alcohol (Macron FineChemicals) for approximately 20 s, and placed in a refrigerator atapproximately 10° C. for several minutes to allow the samples to reachbelow the melting point of the bulk alloy. The samples were then rinsedof any free thiol by centrifugation (RevSpin RS-200). Namely, thesamples were spun at 10,000 rotations per minute (RPM) (5529 relativecentrifugal force (RCF)) for 10 min. The resulting aliquots werediscarded and the pellets were resuspended via light sonication inethanol. This process was repeated three times to ensure there was nounbound thiol in the samples. Next, the resuspended pellets were spun at2,000 rpm (220.5 RCF) for 10 min. The aliquots from this step werepoured into centrifuge tubes to obtain smaller particles, whereas thepellets were resuspended via light sonication in fresh ethanol. Thisprocess was repeated three times. This procedure was further applied tothe aliquots to obtain particles at 4,000, 6,000, and 8,000 rpm (884,1990, and 3537.5, RCF, respectively) in series. Samples from eachdifferential centrifugation condition were deposited on clean Si forscanning electron microscope (SEM)-based particles sizing. Ethanol/watermixtures were made by adding a known volume (Lagnet BioPette, 20 μL) ofethanol to a pellet of dried particles, followed by light bathsonication for resuspening, followed by the addition of the appropriatevolume (Lagnet BioPette, 20 μL) of distilled water to reach the requiredmixture ratio (for this study an ethanol mass fraction of ≈28% wasused).

The polydimethylsiloxane (PDMS, Sylgard 184, Dow Corning) substratesamples were prepared as follows. The polymer is provided as a two-partsystem consisting of a base and a curing agent. The two liquids aremixed in a 10:1 mass ratio. The two parts were mixed by hand and placedin THINKY ARE 310 orbital mixer for further mixing and degassing. Oncemixed, the liquid polymer was cast onto 2 in.×3 in. borosilicate glassslides using a Specialty Coating Systems Spin-coat G3-8 spin coater at200 rev·min⁻¹ for 60 s. Coated slides were cured in an incubator at 60°C. for at least 4 h. Upon curing, the polymer film was cut into 0.25 insquares, and cleaned by sonication in acetone, followed by rising withacetone, isopropyl alcohol, ethanol, and water. The rinsed samples werethen dried with compressed air. PDMS squares were removed from the dicedsample and placed on a clean glass carrier slides for plasma treatment.Slides were treated with oxygen plasma (Plasma Etch Venus 25 plasmaetcher) for 5 min at 50 W with an oxygen flow rate of 10 CFM·min⁻¹.Treated surfaces were used within 24 h to maintain consistent dropwetting and contact line pinning behavior. The Si substrates employedfor drop-depositing were prepared using a procedure described elsewhere.Once received, they were cleaned via bath sonication in acetone,isopropyl alcohol, then ethanol, for 5 min each, drying with air aftereach sonication treatment.

All drop evaporation experiments were conducted at room conditions witha measured temperature (Fluke 87 V with K-type thermocouple) of 23±2° C.and a measured relative humidity (Kele HS-2000D) of 21%35 2%. Typicalexperiments for conducting drop evaporation were conducted as follows.Sessile drops were formed by gently placing small volumes of colloidaldispersion (see previous Experimental Subsections) via pipette (LagnetBioPette, 20 μL) onto stationary substrates located on a microscopestage (Zeta 20 with custom tilt attachment). The substrates employed forobserving the evaporation process were plasma treated PDMS (seePreparation of Substrates). Upon deposition of drops, top and side viewswere filmed from the optical microscope with a screen capture software(Camtasia 8). Using an in-house MATLAB image analysis script, thegeometry of the profiles were analyzed throughout the evaporationprocess until the profile became too small for the script to detect(between 4° and 9°, depending on initial drop size, objective, andfocusing conditions). Films by the process of depositing drops andevaporating them were also made on Si substrates.

Samples for particle size characterization were prepared by depositingsmall volumes of each differential centrifugation condition viamicropipette (Lagnet BioPette, 20 μL) onto clean Si substrates. SEMimages of these samples were then obtained (Philips XL-40 FEI). Allimages were analyzed using previously reported procedure 17 (resultsfrom this process can be found in Figure S4). Topography measurements ofthe self-assembled films on PDMS substrates were obtained using aconfocal microscope (LEXT 3000) with a 100× microscope objective. ThePDMS squares were then mechanically peeled from the carrier glass slideand placed onto smaller glass slides for spectrophotometry measurements(PerkinElmer Lambda 950, light scan from 300 to 820 nm, with aresolution of 0.5 nm and a 5 s dwell time). Following topography andspectrophotometry measurements, the self-assembled films on PDMS wereprepared for SEM (FEI Nova nanoSEM FESEM) imaging via platinumsputtering (Cressington 208 HR, 40 mA, 0.08 mbar for 60 s).Self-assembled films from the deposited drops were also placed on Sisubstrates using the same method as the drop evaporation experiments andimaged via SEM (FEI Quanta 2D FEG Dual-beam SEM).

Langmuir Trough Experiments were conducted to characterize thedependence of surface tension on the areal concentration of EGaInNPs Theinstrument used in this work is a Kibron microtrough with a pair ofautomated movable barriers, which compresses the nanoparticles that arespread on a water surface, and a surface pressure sensor, which controlsthe barriers. 30 Monolayers of EGaINPs were prepared using a spreadingethanolic solution with a concentration of 50 mg·mL⁻¹. The sample wasspread carefully using a positive displacement pipette at a rate of ≈0.6μL·s⁻¹. After the solvent evaporated, the hydrophobic dodecanethiol(1ATC9)-stabilized EGaInNPs remained on the water surface, appearing asa shiny silver color (see Figure S3), and were then compressed by movingthe barriers at a speed of 10 mm·min-1. The surface pressure isothermwas recorded throughout the compression. The temperature of the doubledeionized water (18 MΩ·cm in the trough) was about 20° C.

Evaporation studies reveal two mechanisms of this self-assembly process,as summarized below. FIG. 2A shows profile images of sessile drops withvarious particle concentrations at different times (shown in seconds)during the evaporation process. Referring to FIG. 2A, four sets ofimages are shown, one for each concentration as labeled. Theconcentration shown are in grams of EGaInNP per liter of dispersion. InFIG. 2A, vertical dashed lines are used to highlight that the dropsremain pinned (i.e., constant contact radius) throughout this process.Concentrations are given in grams/liter (g/L). Scale bars are 150 μm inlength. Kinetics of drop geometry during evaporation are illustratedthrough several plots shown in FIGS. 2B through 2E. FIGS. 2B through 2Care plots of contact angle and volume of the drop against evaporationtime respectively. FIGS. 2D and FIG. 2E are plots of dimensionlesscontact angle dimensionless volume against time normalized by drop sizeand initial contact angle. In FIGS. 2B through 2E vertically orienteddashed lines mark the total evaporation time for each experiment. InFIGS. 2D and 2E, the curves labeled “theory” were generated based onobserved initial conditions for the 0 g/L drop experiment. The particlesin the dispersions studied on which FIGS. 2A through 2E are based had amean size of 268 nm with a standard deviation of 51 nm.

It can be seen from FIGS. 2B through 2E that the evaporation dynamicsinitially favor that of pure ethanol despite the fact that the initialconcentration of water is much larger (initial mole fractions of waterand ethanol within the drop are x_(w)≈0.87 and x_(e)≈0.13,respectively), which supports previous experimental observations thatthe drop rapidly segregates during evaporation into an outer ethanolshell and a water enriched core. The evaporation process can be modeledas quasi-steady state, where liquid vapor equilibrium applies. As aresult, the vapor mole fraction of ethanol just above the drop surface,y_(e), is significantly larger than the liquid mole fraction of ethanolwithin the drop (e.g. initially for this study x_(e)≈0.13 andy_(e)≈0.49). To maintain this condition, an excess of ethanol must bepresent at the interface. Moreover, the larger evaporation rate ofethanol during this process amplifies the transport of the ethanol tothe surface. Because it is known that the thiol-capped nanoparticlesform stable colloidal suspensions in ethanol and are also hydrophobic,they migrate to the drop surface via the evaporating ethanol due tofluid-particle interactions, completing the first part of the assemblyprocess. Once at the interface between the water enriched core and theethanol shell, it is expected that the nanoparticles assemble due toparticle-interface interactions, thereby concluding the second phase ofthe hybrid self-assembly process.

The effects of particle concentration on the evaporation process werealso examined in studies leading to this disclosure. It should be notedthat nanoparticle concentrations here are reported per volume of totalsolution). FIG. 2A shows that the drops remained pinned throughout mostof the evaporation process, regardless of particle loading. FIG. 2Bshows similar initial contact angles among all concentrations,indicating that the wetting behavior is dominated by the interactionbetween the solvent mixture and the substrate. FIGS. 2B and 2C show goodagreement between the predictions of dual-solvent evaporation modelsdeveloped in this study. Normalizing the data from FIGS. 2B and 2C toaccount for differences in drop size and shape enables comparisonsbetween the model and drops of various particle loadings. Thesenormalization results are shown in FIGS. 2D and 2E. After accounting fordifferences in drop size and shape, the dynamics appear to behave inaccordance with that predicted from a mixture evaporation theory,indicating that particle concentration has little effect on theevolution of the drop profile during evaporation throughout a majorityof the drop lifetime. On the other hand, subtle differences do appeartoward the end of evaporation, as evidenced by the slight discrepanciesin normalized total evaporation times. For low particle concentrations(≤6.8 g·L⁻¹ or ≤1.08·10¹⁴ particles·L⁻¹) there is no distinguishabledifference. However, a medium concentration (10.7 g·L⁻¹ or 1.70·10¹⁴particles·L⁻¹) exhibits an extended normalized droplet lifetime,indicating that the concentration of the non-volatile nanoparticles islarge enough to change the effective vapor pressure of the drop. Furtherincreasing the concentration (15.0 g·L⁻¹ or 2.38·10¹⁴ particles·L⁻¹)shows a shorter normalized drop lifetime compared to thenanoparticle-free case, indicating a trade-off between effective vaporpressure lowering and the reduced amount of volatile material initiallypresent in the drop. In other words, at high nanoparticleconcentrations, the relative amount of solvent initially present in thedroplet is small enough to offset the reduced effective vapor pressureof the drop, resulting in a shorter normalized droplet lifetime.

FIGS. 3A and 3B summarizes the influence of initial particleconcentration on the final morphology of the film. FIG. 3A shows opticalmicrographs (left) of films left behind (scale bars are 150 μm inlength) with corresponding detail confocal intensity micrographs (right)of the film edge (scale bars are 10 μm in length). Films with highinitial concentrations (15.0 g·L⁻¹ or 2.38·10¹⁴ particles⁻¹) exhibit adense accumulation of particles at the boundary (i.e., a large coffeering), with a uniform film at the interior of the drop. Note thepresence of cracks in the coffee ring of the high concentration film,indicative of high particle loading in dry colloidal deposits. FIG. 3Bshows profiles of the films corresponding to the concentrations shown inFIG. 3A. It can be seen form FIGS. 3A and 3B, that reducing the initialconcentration results in thinner thickness of the uniform interior ofthe film as well as a size decrease of the coffee ring, with a completeelimination corresponding to concentrations <8.8 g·L⁻¹ (1.40·10¹⁴particles⁻¹). Small voids in the film at the smallest initialconcentration (7.3 g·L⁻¹ or 1.16·10¹⁴ particles⁻¹) suggest that theoptimal initial concentration (i.e., the concentration that will resultin a uniform monolayer film) lies between 7.3 and 8.8 g·L⁻¹. This rangecan be transformed to the number of particles in a drop (n) under thespherical cap approximation via n=2cR³ (1+ cos(θ₀))2 (2+ cos(θ₀))/d³ ρsin 3 (θ₀); where c is the initial mass concentration of particles, R isthe areal contact radius of the deposit, θ₀ is the contact angle of thedrop at the onset of evaporation, d is the particle size, and ρ is themass density of the particle. Substituting in appropriate values forthese experiments: 7.3 g·L⁻¹<c<8.8 g·L⁻¹, R≈1.2 mm, θ₀≈25°, ρ=6250kg·m⁻³, and d=268 nm (the particle size used for the study in FIG. 3),results in an optimal number of particles in a drop to be between7.1×10⁷ and 8.5×10⁷. Theoretically, a closely packed monolayer ofparticles from a deposited drop requires n_(optimal)=4pR²d⁻², wherep≈0.907 is the hexagonal packing factor for circles in a plane. 26 Weassume hexagonal packing because the time scale packing because the timescale for the particles to assemble by particle diffusion is found to besmaller than the time scale for the particles to reach the drop surfacedue to evaporation indicating that the likely arrangement of theparticles at the surface is hexagonal packing. Therefore, thetheoretically estimated number of particles per drop to produce uniformmonolayer films is n_(optimal)=7.3×10⁷ (corresponding to an optimalconcentration of c_(optimal)=7.5 g·L⁻¹), which indeed falls within ourobserved range. This demonstrates a process for determining appropriateparticle concentrations for uniform monolayer films in multiple-solventor dual-solvent formulations.

The results described above also indicate that exceeding the appropriateconcentration range will result in the formation of a coffee ring at thecontact line and an increase in thickness over the uniform interior ofthe film, which is expected due to the nature of monolayer growth duringevaporation. Once a monolayer is achieved during evaporation, anyparticles remaining within the drop will begin to form multilayerstructures as they move to the surface and collide with particles withinthe formed monolayer. The likelihood for collisions is largest at theedge of the drop because this is where the radial velocity is highest,resulting in the presence of coffee ring films for c>c_(optimal), asshown in FIG. 3A. Note the appearance of gray spots in the opticalmicrographs of FIG. 3A for c>c_(optimal). These gray spots in theoptical microscope images of the films are a result of differences inlight scattering, indicating differences in local film thickness (lowerlocal thicknesses in the gray areas). This is believed to be a result ofmultiple layers being formed throughout areas of surface while the firstlayer is still being formed in other areas.

Experiments were conducted to demonstrate application of thisself-assembly process to the fabrication of high performancedrop-on-demand thin film optical devices. FIGS. 4A and 4B arequalitative examples demonstrating use of films of this disclosure asthin film devices. FIG. 4A illustrates the application of the films ofthis disclosure as filters, where the film reduces the amount of lighttransmitted through a close-up image of Einstein (left part of FIG. 4A)and a close-up the letter “P” (right part of FIG. 4A). FIG. 4Billustrates they application of the films of this disclosure as mirrors,where the film reflects light incident on (left part of FIG. 4B) animage of an array of “Einsteins” and (right part of FIG. 4B) an image ofan array of letters, “P's”. Films in both these instances (filter andmirror) are approximately 2 mm in diameter. FIG. 4C is a representativeSEM image of film morphology for the 84±18 nm samples used in FIGS. 4Aand 4B. The scale bar in FIG. 4C represents 500 nm in length. Therepresentative scanning electron micrograph in FIG. 4C reveals thatthese films are uniform and densely packed. FIGS. 4D through 4G show themeasured optical results (for various assembled films of variousparticle sizes d in nm, namely absorbance, transmittance, reflectanceand specular reflectance normalized by total reflectance respectively.The larger particles will produce thicker samples, resulting in anincreased absorbance, as seen in FIG. 4D. This gives rise to the reverseeffect for the transmittance as seen in FIG. 4E. The absorbance andtransmittance spectra also flatten with increased particle size. Thismay be due to the increased size heterogeneity with respect to particlesize, allowing for cancellation of resonance peaks associated withdifferent sized particles within the sample. Most notable of this effectis the flat transmission response (13%±2%) associated with the largestparticle (d=268 nm) monolayer films over the entire sampled range (300nm-820 nm), which demonstrates an application of these films as abroadband neutral density filter. As shown in FIGS. 4F and 4G, thesamples are also inherently reflective given their metallic composition.The decrease in particle size gives rise to smoother films. As the filmsbecome smoother, their total and specular reflectance increases,resulting in the trend seen in these figures. As shown in FIG. 4G, FIG.4g , the high specular reflectance response (>80%) associated with thesmall particle (d=84 nm) films shows promise for use as a mirror,especially for wavelengths in the visible-near infrared range.

Experiments leading to this disclosure have revealed that uniform filmsfrom evaporating drops from dual-solvent dispersions are created by ahybrid of two self-assembly mechanisms: 1) Fluid-particle interactions,where the nanoparticles are carried to the drop surface by the highervapor pressure solvent, followed by 2) Particle-interface interactionsonce the particles reach the interface of the phase-segregated drop. Asobserved, this assembly process occurs at the surface of the drop,indicating independence of this mechanism with respect to substrate. Thedual-solvent evaporation model developed as part of this work accuratelydescribes the drop profile kinetics and could be used for future designof dual-solvent based functional inks. The agreement of the size andshape normalized data and model show scalability of this process.Results of experiments leading to this disclosure also show a simplemeans for designing particle concentrations to produce uniformmonolayers. Moreover, the application of dual-solvent formulation tohigh performance drop-on-demand thin film devices such as mirrors andbroad band neutral density filters has been demonstrated. Adaptation ofthe methods of this disclosure to high throughput drop depositionmethods (e.g. roll-to-roll inkjet printing) can prove to be viable meansto fabricate thin film devices.

Thus in this disclosure a method has been demonstrated for producingstrikingly uniform films from evaporating drops contained in adual-solvent dispersion. This assembly process leverages bothparticle-fluid interactions to carry the particles to the drop surfaceand particle-interface interactions to assemble the particles into auniform film. The process can be employed to produce thin film devicessuch as flexible broadband neutral density filters and semi-transparentmirrors. Experimental results indicate that this assembly process isfree of particle-substrate interactions, which indicates that theresults should be transferable across a multitude of material/substratesystems. Further, the concepts and approaches can be extended tomulti-solvent dispersions as explainer earlier.

Based on the above studies and discussion, it is an objective of thisdisclosure to describe a method of depositing uniform films of particleswith monolayer capability wherein the method includes a drop evaporationprocess resulting in a film comprising a monolayer.

Further, it is an objective of this disclosure to describe a method ofproducing a film comprising particles. The method includes the steps ofproviding particles with a coating on their outer surface such that theparticles are substantially non-dispersible, substantially non-solubleand substantially non-suspendable in a first liquid, but aresubstantially dispersible and/or substantially soluble or substantiallysuspendable in a second liquid. Further, the first liquid and secondliquid are miscible and the evaporation rate of the second liquid isgreater than the evaporation rate of the first liquid. The particles aredispersed or dissolved or suspended as the case may be into the secondliquid. The resulting mixture is further mixed with the first liquid toform a process mixture. The process mixture is then deposited onto asubstrate forming a deposit, and the first liquid and second liquid areevaporated from the deposit resulting in a film comprising the particleson the substrate. In one embodiment of the above mentioned method, thevapor pressure of the second liquid is greater than the vapor pressureof the first liquid. In another embodiment of the method, diffusioncoefficient between the second liquid and ambient atmosphere is greaterthan diffusion coefficient between the first liquid and the ambientatmosphere.

In the above method. Many choices exist for the first liquid. In oneembodiment, the first liquid is water and the second liquid is anorganic liquid, which, in some cases, can be an organic solvent. In oneembodiment of the method, the particles have a coating on their outersurfaces. In one embodiment, the particles are made of a metal and acoating on the outer surface of the particles comprisessulfur-containing molecules. In a preferred embodiment thesulfur-containing molecules are molecules of a thiol. A non-limitingexample of a thiol is 3-mercapto-N-nonylpropionamide (trade name 1ATC9from Sigma-Aldrich). Alternatively, the sulfur-containing molecules canbe molecules of an ionic liquid. A non-limiting example of such an ionicliquid is the ionic liquid is Methyltrioctylammonium thiosalicylate. Itshould be recognized that the coating can contain both sulfur-containingmolecules and non-sulfur containing molecules.

In a preferred embodiment of the above method, the first solvent iswater and the second solvent is ethanol. The particles employed in thispreferred embodiment are thiol-coated eutectic Gallium-Indium (Ga—In)particles (in this case the thiol employed is3-mercapto-N-nonylpropionamide). The thiol coating makes the particleshydrophobic and hence they are substantially non-soluble, substantiallynon-dispersible and substantially non-suspendable in the first solvent,namely water in this preferred embodiment. The second solvent in thispreferred embodiment is ethanol and the thiol coated eutectic Ga—Inparticles are soluble and dispersible in ethanol. Further ethanol has ahigher vapor pressure than water. This dispersion can be deposited on asubstrate, which in a preferred embodiment is polydimethylsiloxane(PDMS). As described above in the experiments and analysis of theexperimental results, evaporation of the solvents leads to a film of theparticles. In a preferred embodiment, the particles formed comprises amonolayer. In the method described above the resulting films are uniformi.e the film has a surface roughness in the same order of magnitude asthe particle size of the particles. In one embodiment of the methoddescribed above the particles are made of a GA—In alloy, and further ina preferred embodiment, the Ga—In alloy has eutectic composition. In oneembodiment of the method described above, the particles have a sizerange of 1 nm to 2000 nm, with a preferred range being 100 nm to 250 nm.It should be recognized that in certain instances the uniformity of thefilm is found in a large segment of the film, but sometimes notnecessarily at the edges of the film. The edges can have higherthickness due to the evaporation rates, concentrations of the particlesin the dispersions etc. as described above. It should be also be notedthat while in most cases evaporation is allowed to occur under theambient conditions, it is possible to control temperature, humidity andpressure to optimize the evaporation rate to obtain desired rate ofevaporation.

In the method described above, many techniques can be used to depositthe process mixture onto a substrate. These techniques include, but notlimited to drop casting, spin coating, doctor blading, aerosoldeposition (a process by which particles in an aerosol collect ordeposit themselves on solid surfaces), slot die process (known to thoseskilled in the art of depositing coatings and films), microgravuredeposition techniques (discussed in the literature), direct writing, andinkjet printing.

In the methods describe above, several substrate choices exist. Examplesthat can be used include but not limited to polydimethylsiloxane (PDMS)and silicon.

It should be noted that in a variation of the method, the particlesinclude a first type of particles, and a second type of particles,wherein the first type of particles are capable of dissolving and/ordispersing in the first liquid and the second type of particles arecapable of dissolving or dispersing in the second liquid. In this case,the process mixture comprises two liquids and two types of particles andthe film formed has one layer substantially made of the first type ofparticles and a second layer substantially made of the second type ofparticles. In one embodiment of this method with two types of particles,the first type of particles are capable of dissolving and/or dispersingin the first liquid and the first liquid is phobic to the second type ofparticles; and further, the second type of particles are capable ofdissolving and/or dispersing in the second liquid and the second liquidis phobic to the first type of particles. In this instance, one expectsformation of two substantially distinct layers, when the vapor pressuresof the two liquids are significantly different. It should be noted thatif both the two types of particles are substantially soluble anddispersible in both the liquids, the resulting film from the method ofthis disclosure will be a particulate film wherein both types ofparticles may be intermixed depending on vapor pressures of the twoliquids and the concentrations of each type of particles in thedispersion.

It is possible to modify the above methods to create more than twolayers with more than two different types of particles made ofmaterials. For example we can use three liquids, liquids 1, 2 and 3,such that material 1 dissolves or is dispersible in liquid 2, whichevaporates faster than liquid 1, material 2 dissolves in solvent 3,which evaporates faster than liquid 2. The result is a layered structurewith the bottom layer being substantially composed of material 1, thenext layer substantially composed of material 2, and a third layer beingessentially composed of material 3. It should be noted that thisapproach can be extended to more than three materials and three liquids.In this method it is understood that the particles, dissolving in onesolvent are not soluble or dispersible or suspendable in the othersolvents. As explained above, for the case of two liquids and two typeof particles, it should be recognized that in cases where 3 or moretypes of particles.

It is an objective of this disclosure to describe films made by theabove method, namely particulate films of uniform thickness comprisingone or more layers, each layer substantially made of a particular typeof particles. The films of this disclosure comprise particles andpossess edges that are thicker than the interior, approximately two toten times thicker than in the interior depending on the number of layersof particles in the film at these edge locations due to the process ofthe film formations as described earlier. Another embodiment of thisdisclosure is a particulate film comprising one or more layers ofparticles, each layer substantially made of particles of chemicalcomposition different from the chemical composition of particles ofother layers, wherein the film having a greater thickness at its edgethan in an interior region of the film. In the above cases the edge isto be understood to mean typically about 10-20 micrometers from wherethere is no particles are formed on the substrate and this higherthickness is found in that region meaning that in regions closest to thearea where one observes no particles are formed, the thickness issimilar or less than in the interior of the film.

It should be noted that the chemical composition of the particles is tobe chosen consistent with the application in mind. For example, asdescribed in this disclosure, particulate films whose particles have thechemical composition as thiol-coated eutectic Ga—In alloy were shown tobe suitable for certain optical devices, such as, but not limited tooptical filters and semi-transparent mirrors. Thus the chemicalcompositions of the particles has to be chosen keeping in mind theapplication of a device that would exploit the physical, optical,electronic, biological or other properties of the film for devices suchas sensors.

It should be noted that while the above description describes use ofmetallic particles, the method and the films of this disclosure are notlimited to metallic particles. Polymer particles can be used in themethod resulting in polymer films. It should also be noted thatbiological materials can be used in the method resulting in biologicalfilms.

It is a further objective of this disclosure to disclose several devicesand applications utilizing the films and methods of this disclosures.Non-limiting example of such devices and applications includeoptoelectronics devices, photovoltaic devices, and electronic devices.

It is an objective of this disclosure to describe an optical devicecomprising a particulate film containing at least one layer ofparticles, wherein the at least one layer is substantially made ofparticles of a chemical composition and has uniform thickness. In oneembodiment of the optical device the chemical composition isthiol-coated eutectic gallium-indium alloy. Particulate films whereinthe particles are of thiol-coated eutectic gallium-indium alloyparticles have been demonstrated in this disclosure to be suitable as anoptical filter wherein the film reduces the amount of light transmitted.Another non-limiting example of a device comprising a particulate filmwherein the particles are thiol-coated eutectic gallium-indium alloyparticles is a semi-transparent or semi-opaque mirror as described andillustrated above in this detailed description. In some preferredembodiments of the above described filter and mirror, the particulatefilm is a monolayer.

While the present disclosure has been described with reference tocertain embodiments, it will be apparent to those of ordinary skill inthe art that other embodiments and implementations are possible that arewithin the scope of the present disclosure without departing from thespirit and scope of the present disclosure. It is therefore intendedthat the foregoing detailed description be regarded as illustrativerather than limiting. Thus this disclosure is limited only by thefollowing claims.

The invention claimed is:
 1. A particulate film comprising at least onelayer of particles, wherein the at least one layer is substantially madeof particles of thiol-coated eutectic gallium-indium alloy and hasuniform thickness.
 2. The particulate film of claim 1, wherein theaverage film thickness is in the range of 1 nm to 2000 nm.